The present invention relates to a method and composition for gravel packing of well bores penetrating subterranean formations and particularly for gravel packing highly deviated well bores.
Unconsolidated formations, particularly those containing loose sands and soft sandstone strata, present constant problems in well production due to migration of loose sands and degraded sandstone into the well bore as the formation deteriorates under the pressure and flow of fluids therethrough. This migration of particles may eventually clog the flow passages in the production system of the well, and can seriously erode the equipment. In some instances, the clogging of the production system may lead to a complete cessation of flow, or "killing" of the well.
One method of controlling sand migration into a well bore consists of placing a pack of gravel on the exterior of a perforated or slotted liner or screen which is positioned across an unconsolidated formation to present a barrier to the migrating sand from that formation while still permitting fluid flow. The gravel is carried to the formation in the form of a slurry, the carrier fluid being removed and returned to the surface. The proper size of gravel must be employed to effectively halt sand migration through the pack, the apertures of the liner or screen being gauged so that the gravel will settle out on its exterior, with the slurry fluid carrying the gravel entering the liner or screen from its exterior.
Prior to effecting the gravel pack, drilling mud and other contaminants may be washed from the well bore, and the formation treated. Commonly employed treatments include acidizing to dissolve formation clays, and injecting stabilizing agents to prevent migration of formation components.
While numerous methods are available for effecting gravel packs in substantially vertical well bores, such methods often are unsatisfactory in effecting gravel packing of highly deviated well bores. Conventional gravel packing fluids utilizing uncrosslinked hydroxyethylcellulose, hydroxypropylguar, xanthan gum and the like as the viscosifier allows or permit the gravel to fall or settle to the low side of the tubing in long highly deviated wells. Such settling can result in a premature "sand-out" caused by a bridging of the settled particles across the tubing. Mechanical devices such as a baffled washpipe and oversized washpipes have been used to attempt to prevent or minimize the premature settling of the gravel. Such devices have experienced only limited success in solving the problem.
An ideal fluid for gravel packing operations would be one that shows little or no settling of gravel so that a high concentration of gravel can be transported through the tubing at any angle. The fluid also should exhibit adequate fluid loss to insure compact packing of the gravel against the formation face. The fluid also should "break" to a reduced viscosity fluid similar to the viscosity of water over a predesigned time interval and deposit no residual solids so as to avoid or minimize any formation damage.
Gravel packing of wells which are highly deviated necessitate the use of a viscosified fluid which is capable of providing an infinite gravel fall rate, that is, a zero rate of gravel settling in the fluid during placement in the well bore. A gravel packing fluid having a zero gravel fall rate would assure that the gravel being carried to the production zone in a highly deviated well bore would not prematurely settle-out.
One method employed to substantially achieve a zero gravel fall rate with a polymeric gravel packing fluid is to crosslink the polymer. An aqueous solution of a crosslinked polymer provides a gel structure which suspends the gravel particles causing substantially a zero fall rate. Polymers which typically have been utilized are hydroxypropylguar and carboxymethylhydroxyethylcellulose because of their ease of crosslinking with metal ions such as titanium (IV) and aluminum (III). The main objections to using such fluids for gravel packing operations are the limited control of fluid loss to the formation and the excessive amount of formation damage which can result from use of the fluids.
The present invention provides a solution for, or at least mitigates, the above-discussed problems. The present invention provides improved methods for gravel packing well bores penetrating subterranean formations utilizing a novel crosslinked gelling agent.